1. Field of the Invention
The present invention relates to liquid crystal display devices with a controlled state of the orientation film surface and liquid crystal molecules with different pretilt angles present in admixture in a single picture element, and more particularly it relates to STN (super twisted nematic)-type liquid crystal display devices with excellent viewing angle properties which are widely used as display device means in OA (office automation) devices such as word processors and personal computers.
2. Description of the Related Art
STN-type liquid crystal display devices employ a liquid crystal display element wherein a liquid crystal layer is sandwiched between a pair of substrate members at least one of which is transparent, and the liquid crystal molecules have a twisted orientation of 180.degree. to 270.degree. between the substrate members. The display of the STN-type liquid crystal display devices is colored because of the effect of birefringence of the liquid crystal molecules, and by selecting the placement of the polarizing plate, the display may be colored, for example, "yellowish green" or "blue". In addition, DSTN (double-layered super twisted nematic)-type liquid crystal display devices employ, as a liquid crystal element for color compensation, the liquid crystal element wherein the twisted orientation of the liquid crystal molecules is opposite to that of the above-mentioned liquid crystal display element. Since the coloration produced with the liquid crystal display element is offset by using color-compensating liquid crystal element, an achromatic display is obtained. However, the above-mentioned DSTN-type of liquid crystal display devices have the problem that such display device is thick and heavy because two liquid crystal elements are used.
In order to achieve achromatic displays and thinner and lighter display devices, another type of liquid crystal display device has been proposed, i.e. one which employs a phase difference plate instead of the color-compensating liquid crystal element. Phase difference plates, obtained by uniaxially orienting polymer films, make it possible to produce thinner and lighter liquid crystal display devices. In addition, as the phase difference plate there has been proposed the use of uniaxial oriented polymer films satisfying the relationship nx.gtoreq.nz&gt;ny and having a value of Nz with a range of 0.ltoreq.Nz.ltoreq.0.5, where nx, ny and nz are the refractive indexes in the three-dimensional directions of the film. Here, the value of Nz is a value represented by Nz=(nx-nz)/(nx-ny). Thus, the degree of change of the retardation value of the liquid crystal display element, i.e. the product d.multidot..DELTA.n of the thickness d of the liquid crystal layer of the liquid crystal display element and the refractive index anisotropy of the liquid crystal molecules .DELTA.n, due to the viewing angle, is roughly identical to the degree of change of the retardation value of the phase difference plate due to the viewing angle, and thus a liquid crystal display device with extended viewing angle properties can be obtained.
In other words, the liquid crystal molecules of the liquid crystal display element have a twisted orientation between the pair of substrate members, while forming a pretilt angle C as shown in FIG. 14. Furthermore, the liquid crystal molecules have a refractive index anisotropy .DELTA.n as mentioned above. This means that liquid crystal display elements have different retardation values with respect to the display device surface depending on the viewing angle orientation (tilt angle orientation) formed upward from the display device surface. The former type of phase difference plates are made of a uniaxial oriented polymer film, and therefore color compensation can only be made in a specific viewing angle direction. However, the latter type of phase difference plate has its refractive index in three directions controlled, which thus widens the viewing angle range to obtain a satisfactory display.
FIG. 14 is a sectional view of the state of orientation of liquid crystal molecules 7a of a conventional liquid crystal display element 8. The above-mentioned pretilt angle C is an angle formed between each of the surfaces of the orientation films 5, 6 in contact with the liquid crystal layer 7 of the substrate members 1, 2 and the longitudinal direction of the liquid crystal molecules 7a closest to the surface of the oriented films 5, 6. Here, the substrate members 1, 2 are each provided with at least the orientation films 5, 6 on, for example, a pair of transparent plates 3, 4.
Additionally, in a STN-type liquid crystal display device, the phase difference plate used is a one having a property of nx.gtoreq.nz&gt;ny, namely, which has a phase difference not only in a plane parallel to the surface of the phase difference plate, but also in the thickness direction. Owing to the phase difference in the plane parallel to the surface of the phase difference plate is compensated during non-select voltage application the color tone of the off state, which intercepts the light during non-select voltage application, and the color change at a viewing angle inclining to the direction parallel to a display surface is compensated by the phase difference thickness direction.
In a TN-type liquid crystal display device wherein liquid crystal molecules are oriented with a twist of 90.degree. between a pair of substrate members, a phase difference plate having a phase difference not in a surface parallel to the surface thereof, but only in the thickness direction, used to compensate the color tone of the normally white mode in which light is transmitted through the phase difference during non-select voltage application.
Thus, the performance requirements are different between the phase difference plates employed for a TN- and a STN-type liquid crystal display device, and accordingly when the phase difference plate to be used for a TN-type one is employed for a STN-type one, a desired effect can not be obtained.
In a STN-type liquid crystal display device, the above-mentioned type of phase difference plates are provided in order to achieve an achromatic display and a thinner and lighter-weight display device, but no color compensation can be obtained in the reverse viewing angle direction of the display device, while what are obtained from the reverse viewing angle are only images with a low-contrast, those with different colors, and light/dark inverted images, in which occurs a so-called reversal phenomenon. This is because the liquid crystal molecules 7a of the liquid crystal display element 8 have the pretilt angle C, and the refractive indexes of the liquid crystal molecules 7a differ when observed from the 6 o'clock direction (normal viewing angle direction) indicated by the arrow 9 and the 12 o'clock direction (reverse viewing angle direction) indicated by the arrow 10.
FIGS. 15A, 15B are illustrative sectional views for a more detailed explanation of the state of orientation of the liquid crystal molecules 7a. FIG. 15A shows the state as seen from the 6 o'clock direction (normal viewing angle direction) indicated by the arrow 9 in FIG. 16, and FIG. 15B shows the sate as seen from the 9 o'clock direction (left viewing angle direction) indicated by the arrow 11 in FIG. 16. The liquid crystal molecules 7a are oriented along the rubbing direction of the oriented films 5, 6 of the substrate members 1, 2, and have the pretilt angle C as mentioned above. They are also oriented with a twist of, for example, 240.degree. between the substrate members 1, 2. That is, the substrate members 1, 2 are arranged so that the liquid crystal molecules 7a are oriented with a twist of 240.degree. therebetween. Here, the longitudinal direction of the liquid crystal molecules 7ac positioned in the almost middle of the substrate members 1, 2 is selected, for example, so oughly parallel to the 6 o'clock-12 o'clock direction. The intermediate molecules 7ac are also tilted at the above-mentioned angle C.
Observing this type of conventional liquid crystal display element 8 from the normal viewing angle direction and reverse viewing angle direction 10, the refractive indexes of the liquid crystal molecules 7a in each of the viewing angle directions 9, 10 are different, and the retardation value is no longer symmetrical in the normal viewing angle direction 9 and reverse viewing angle direction 10. Consequently, with the phase difference plates formed by uniaxial orientation, it is impossible to carry out color compensation simultaneously for both the normal viewing angle direction 9 and the reverse viewing angle direction 10, leading to the inconveniences as described above.
In Japanese Unexamined Patent Publication JPA 5-210099 (1993) is disclosed an example of a TN-type liquid crystal display device wherein the above-mentioned inconveniences are preferably dissolved. In the liquid crystal display device, a part of one at the liquid crystal layer side, of two laminated orientation films with different orientation directions is peeled. The orientation film is generally formed to have a thickness of hundreds A, and according to the publication, a number of irregularities are formed in the surface of the orientation film. In a TN-type liquid crystal display device, irregularities having a thickness of about 100 A adversely affect the image quality only in a small extent, while in a STN-type liquid crystal display device, even such irregularities adversely affect the image quality in a great extent. More concretely, the orientation state of liquid crystal molecules becomes disordered, which causes lowering of contrast ratio.